1. Technical Field
The present disclosure relates to a device for programming a phase-change memory (PCM) cell with discharge of capacitance and to a method for programming a PCM cell.
2. Description of the Related Art
As is known, phase-change memory (PCM) elements exploit the characteristics of materials that have the property of switching between two phases having distinct electrical characteristics. For example, these materials can switch between an amorphous, disorderly, phase and a crystalline or polycrystalline, orderly, phase, and the two phases are associated with resistivities of considerably different values. In addition, intermediate configurations, in which the material has only partially switched to the amorphous phase or to the crystalline phase, can be associated with intermediate values of resistivity.
Currently, the alloys of Group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can be advantageously used in phase-change cells. The currently most promising chalcogenide is formed by an alloy of Ge, Sb and Te (Ge2Sb2Te5, GST), widely used also for storing information in overwritable disks. In chalcogenides, the resistivity varies by two or more orders of magnitude when the material passes from the amorphous phase (more resistive) to the crystalline phase (less resistive) and vice versa.
The use of PCM elements for providing memory cells and arrays has been already proposed. In particular, in phase-change memories, a portion of chalcogenic material is used as programmable resistor, which can be electrically heated by a controlled current so as to switch between a high resistance condition and a low resistance condition and vice versa, respectively associated to which are a first logic value and a second logic value. The state of the chalcogenide can be read by applying a voltage sufficiently low as not to cause a sensible heating and by measuring the current that traverses it. Since the current is proportional to the conductance of the chalcogenic material, it is possible to distinguish the two states.
As has been mentioned, the phase transitions between the highly resistive amorphous state and the highly conductive crystalline state can be induced electrically through current pulses of appropriate amplitude and duration.
In particular, the transition towards the amorphous state (“reset”) is obtained by applying current pulses of an amplitude sufficient to heat the chalcogenide beyond the melting point by Joule effect. The current pulses are generally rectangular, or in any case with steep edges so that cooling of the chalcogenide is so fast as to prevent crystallization.
The transition towards the crystalline state (“set”) can be induced in different ways, and different techniques are currently used.
Irrespective of the technique adopted, a reset pulse is applied preliminarily for rendering completely amorphous an adequate volume of chalcogenide.
A first technique uses rectangular set current pulses. The amplitude of the set pulses is smaller than the amplitude of the reset pulses so that the temperature of the chalcogenide will exceed a phase-switching temperature, without, however, reaching the melting temperature. The duration of the pulses is, instead, sufficient to enable complete crystallization of the chalcogenide. The time required for programming is, however, excessively long, and the levels of performance of the memories are not acceptable.
A second technique, described in U.S. Pat. No. 6,570,784 exploits trapezoidal set pulses. The amplitude of the set pulse is initially close to the amplitude of the reset pulse, and then decreases according to a linear ramp for a time interval, after which the pulse is interrupted. Also the temperature of the chalcogenide decreases (“quenching”) according to a linear ramp. The technique exploits the fact that the crystallization time is very short in a narrow range of temperatures around an optimal temperature. The initial amplitude and final amplitude of the set pulse can be easily calibrated in such a way that the temperature of the chalcogenide, during the quenching ramp, will vary around the optimal crystallization temperature. In the previous case, instead, the duration of the set pulses had to be in any case long enough to enable crystallization even with temperatures of the chalcogenide significantly different from the optimal temperature so as to take into account the process dispersions and the changeable operating conditions.
According to a further solution, a single pulse is supplied having a constant stretch and a decreasing-ramp stretch, instead of a reset pulse followed by a set pulse. The initial amplitude of the pulse is sufficient to bring the chalcogenide to the melting temperature. Once a time interval has elapsed such as to ensure complete amorphization, the amplitude of the pulse decreases according to a linear or discrete ramp. The time necessary for programming is further reduced, since a single pulse is used. However, the quenching time is in any case rather long, since the ramp starts from a temperature higher than the melting temperature. In addition, to produce the set pulse a waveform-forming circuit is used that is rather complex and cumbersome.
According to U.S. Pat. No. 7,075,841, the set pulse has a trailing edge of an exponential type. The proposed solution enables reduction of the time necessary for programming the individual memory cell, but requires two separate circuits for generating the set and reset pulses.